1. Field of the Invention
The present invention relates to a cleaning method to clean and remove fine particles and organic deposits on a surface of a hard disk as a medium and on a silicon wafer, and relates to a method for the same.
2. Description of the Related Art
For a medium such as a hard disk, silicon wafer, and a glass plate for manufacturing a liquid crystal display, minimizing defects on the surface, cleaning the surface completely for the purpose of fine manufacturing processes, and removing fine particles and organic deposits are necessary.
For such a cleaning apparatus for cleaning the surface of a silicon wafer and the like, a cleaning apparatus which ejects a CO2 (dry ice) snow jet at the surface of a workpiece for cleaning has been used. In such a cleaning apparatus, high pressure liquid CO2 is ejected from a narrow nozzle to produce CO2 snow. That is, most of the CO2 which is ejected from the nozzle become liquid condensate. The surfaces of such condensates evaporate and carry away the heat of vaporization from the liquid condensates. The liquid condensates from which the heat of vaporization has been removed solidifies and becomes dry ice such as CO2 snow. The CO2 is jetted at the workpiece to be cleaned, and the surface of the workpiece thereby cleaned. The size of the particles of the CO2 snow mostly depends on the size of the liquid condensate droplets
In addition, instead of liquid CO2 at normal temperature, a cleaning method in which the liquid CO2 is cooled to the triple point and a mixture of solid, liquid, and gas is ejected for cleaning is proposed. Liquid CO2 cannot exist at temperatures lower than the triple point, and the liquid CO2 immediately solidifies. Also, a part of the gas is cooled down and coheres and solidifies. In this case, the size of the particles of the CO2 also depends on the size of the liquid condensate droplet.
Furthermore, a method in which liquid argon instead of CO2 is cooled down to the triple point, a mixture of solid, liquid, and gas argon is ejected from a nozzle for cleaning has been performed. In this case, the triple point of argon is lower than atmospheric pressure; thus, there is a limitation in such a cleaning method must be performed in a container in which gas pressure is reduced.
In these cleaning process, condensation of moisture in air occurs on the surface of the workpiece when atomspheric air contacts the workpiece to be cleaned, because the workpiece is cooled by CO2 snow. In order to avoid such a case, a stage for mounting the workpiece to be cleaned must be heated by a heater. In a cleaning method in which argon is used, the workpiece to be cleaned must be heated in order to avoid the occurrence of condensation of moisture in air when the workpiece to be cleaned returned to the atmosphere. Also, in order to decrease moisture in the surrounding atmosphere, the region for performing the cleaning operation was sealed, and low dew point gas was filled therein. Furthermore, static electricity was removed by an ionizer so as to prevent the build up of static electrical charge.
However, there were problems in such cleaning apparatuses as follows.
In a cleaning apparatus which used CO2 snow, when particles such as those dust exist in the liquid CO2, such particles may impair the workpiece to be cleaned. This was a problem because it was difficult to remove such particles.
Such particles can be removed by disposing a filter in pipes for conducting the liquid CO2. However, if the pores of the filter are too fine, pressure loss increases when passing through the filter; thus, dry ice is formed downstream of the filter, and it was thereby not possible for the filter to be made with finer pores. For this reason, the fine particles could not be removed.
To deal with this problem, extremely pure liquid CO2 was used in a conventional cleaning apparatus; however, such a cleaning apparatus is extremely expensive operate.
Also, the size of the dry ice snow which are ejected from the nozzle depends on the diameter of the nozzle and the flow rate. However, it is difficult to make a nozzle of smaller diameter due to machine precision limitations; thus, it was also difficult to produce smaller dry ice snow. For this reason, there is a problem in that the cleaning of the workpiece is difficult.
In addition, the cleaning apparatus becomes complicated because it is necessary to retain a workpiece to be cleaned in a sealed container and in a vacuum container when CO2 or argon is used. Thus, there is a problem in that the cost of the apparatus is high.
Furthermore, if solid or liquid cleaning solvent impacts the workpiece to be cleaned, static electrical charge is generated due to the friction with the surface of the workpiece to be cleaned; and thus, particles which were removed once adhere again because of such static electrical charge, and there are cases in which static electrical charge occurs to devices such as FETs (field effect transistor) formed on the surface. There was also a problem in that it was difficult to completely remove static electrical charge by an ionizer.